Checksum watched files by repo-relative path

[omnibs]

A cache built in one checkout can never be fresh in another: Checksum serializes each watched file's absolute path, so identical trees at different paths (e.g. linked git worktrees warmed by cloning _build/firstaide from the main checkout) get a different cache.<sig> name and fail the sums::equal freshness check. Result: "Nix environment is out of date! … Loading STALE environment" on every single load, even when the cache is byte-for-byte current.

This stores checksum paths relative to the build directory instead. Content hashes are unchanged. The hook absolutizes the paths against the build dir when emitting watch_file lines, so direnv still watches the right files wherever the cache came from. Paths outside the build dir stay absolute.

Old caches degrade gracefully: they still load through the cache symlink fallback, compare stale once, and the next firstaide build writes a portable cache. Bumped the version to 0.1.7 since existing cache signatures all change.

Verified with a synthetic project — cache built in tree-a, tree copied to tree-b at a different path:

--- OLD binary (firstaide 0.1.6) in tree-b:
Environment is STALE!
--- NEW binary (firstaide 0.1.7) in tree-b:
Environment is up to date!

and the upgrade path (new binary on an old-format cache): stale once, up to date after one rebuild.

[Copilot]

Pull request overview

This PR makes the checksum (and derived cache signature) portable across different checkouts of the same working tree by storing watched file paths relative to the build directory, eliminating stale-cache behavior when the repo is located at a different absolute path.

Changes:

• Make Checksums/Checksum serialize watched paths relative to a provided root directory (keeping paths outside the root absolute).
• Update commands that compute checksums to pass config.build_dir as the root and update the hook to absolutize stored paths when emitting watch_file entries.
• Bump crate version to 0.1.7 and add unit tests covering root-relative and outside-root path behavior.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/sums.rs	Store checksum paths root-relative and add tests ensuring portability across different roots.
src/config.rs	Expose abspath so other commands can absolutize stored (now-relative) watch paths.
src/cmds/status.rs	Compute checksums using a root (build dir) for consistent signatures.
src/cmds/hook.rs	Absolutize cached watch paths when generating direnv watch_file directives.
src/cmds/build.rs	Compute checksums using a root (build dir) so cache keys don’t depend on absolute checkout location.
Cargo.toml	Bump package version to 0.1.7.
Cargo.lock	Update locked package version to 0.1.7.

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